Fluid delivery systems have a wide variety of applications and are commonplace in many household appliances, such as metered water dispensers, icemakers, dishwashers, and clothes washers. In such systems, the volume of the fluid to be dispensed is typically controlled by a timing mechanism operable to open and then close a valve after predetermined period of time has elapsed.
These fluid delivery systems operate under the presumed condition that the fluid line pressure and, correspondingly, the fluid flow rate are known and static. All too often, however, the fluid delivery systems are susceptible to fluctuations in the fluid line pressure, and resultantly, the fluid flow rate varies. In such instances, when the fluid line pressure is lower than expected, an under-fill condition typically occurs. Conversely, when the fluid line pressure is greater than expected, an over-fill condition may result.
Consequently, it is desirable for fluid delivery systems to include an in-line sensor that monitors the fluid flow rate and enables the system to accordingly compensate the fluid delivery time.
Another desirable feature for a fluid flow sensor is the absence of any moving parts, such as a paddle-wheel or a turbine, for example. This is because there exists the probability that contaminants, debris or other small particle matter may be present in the fluid. Its function independent of moving parts, the fluid flow sensor is thereby more reliable under such operating conditions.
Thermo-anemometers are a type of flow rate sensor that does not require any moving parts to operate. Thermo-anemometers function based on the principles of heat transfer. Such flow rate sensors, though well-known, have traditionally lacked the necessary response times to make them suitable for many common applications, like water delivery systems for household appliances.
Traditional thermo-anemometers typically include two temperature sensors: one temperature sensor disposed at a downstream location in the fluid path for measuring the fluid temperature downstream; and another temperature sensor disposed at an upstream location in the fluid path for measuring the fluid temperature upstream. The sensor measuring the upstream temperature compensates for fluctuations in the water temperature that might bias the reading of the sensor measuring the downstream temperature. The thermo-anemometer subtracts the upstream temperature from the downstream temperature. By employing various known equations and thermal sensing principles, such as the Seebeck Effect, the temperature difference may be correlated to a fluid flow rate.
Other techniques for determining a fluid flow rate that are fundamentally based on thermodynamic principles are also known. For example, one method is to measure the heat loss, over time, of a known heat source that exposed in the flow of the fluid. The heat loss, expressed as a temperature drop, can be correlated to a fluid flow rate. This method, however, can take a relatively long time period to provide usable results. The reason is primarily because it can take several seconds for the temperature of the source to reach steady-state after being exposed to the fluid flow. In some applications, however, such as in an icemaker, for example, this response time is much too slow.
FIG. 1, illustrates an exemplary graph of a typical response curve (temperature (T) vs. time (t)) of one such known system. When temperature (T) is plotted versus time (t), it is seen that the steady state value for temperature is not reached for about 1.5 to 2.0 seconds after the fluid flow is initiated. In the example illustrated, temperature was measured under water flow rate conditions of both 0.15 gallons per minute (GPM) and 0.75 GPM.
The need remains, therefore, for a fluid flow sensor that does not involve any moving parts, has a faster response time, and is easily integrated into a variety of fluid delivery systems.